Sandstone forms when tiny sand grains bind together into a solid mass. The edges of sandstone slabs wear away when exposed to the elements, leaving behind spectacular structures such as arches, columns and alcoves that resist further erosion.

Geologist Jiří Bruthans of Charles University in Prague was touring a sandstone quarry when he noticed something odd: The workers had to use explosives to break apart the solid sandstone walls, but rocks that broke free often quickly crumbled apart. Bruthans says this behavior seemed to contradict the conventional explanation that chemical cement glues sandstone structures together. Thinking another overarching force was responsible, Bruthans decided to play in the sand.

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